The present invention relates to a method and apparatus for controlling downhole tools within a borehole which tools have been conveyed into the borehole on production tubing, and more particularly for controlling a downhole perforating gun.
In order to extract hydrocarbons, both crude oil and natural gas, from earth formations it is neccessary to drill a borehole into such formations. This borehole is typically lined with a tubular steel casing in order to prevent the borehole from caving in and blocking access to the formations, as well as to prevent the uncontrolled flow of the hydrocarbons, or the intermixing of the hydrocarbons from various zones, or the contamination of these hydrocarbons with undesirable minerals or substances located within the formations surrounding the borehole. The casing is cemented to the adjacent formations surrounding the borehole to minimize any erosion of those formations which might create flow paths exterior to the casing. In order to establish communication with the producing zones of the formation, it is therefore necessary to perforate the casing and the surrounding cement to provide a path for the hydrocarbons from the formations into the well casing. This perforating is typically done with explosive charges which when actuated fire a high energy jet that passes through the casing and into the formation to create a drainage path for the hydrocarbons. Typically, one or more of these explosive charges are rigidly mounted in a predetermined pattern on a support strip, which support strip can be lowered into the borehole to a position adjacent formations which are to be perforated. Since the ambient conditions within the borehole involve extremely high temperatures and pressures (for example, 290.degree. C. and 2000 bars pressure) it is often desirable to protect the support strip and the charges contained therein with a hollow tubular steel casing often referred to as a charge carrier. This combination, referred to as a perforating gun, can often be several meters long and weigh in the vicinity of 2500 kilograms. It is therefore not a negligible matter to convey this perforating gun into and out of the borehole. Since the perforating charges are not to be actuated until the perforating gun is positioned adjacent the formation which is to be perforated, it is also necessary that some form of communication be provided between the perforating gun and the surface of the borehole through which can actuate the charges only after the perforating gun has been accurately positioned. It has thus been customary to convey the perforating gun into and out of the borehole with a specialty cable known as wireline which is designed to afford adequate strength members to support the gun and the suspended portion of the wireline downhole, as well as communication means, such as electrical conductors or optical fibers to facilitate the necessary communication with the perforating gun in order to selectively actuate the charges contained therein. The practical mechanical limitations of such wireline, which must be adequately flexible to be wrapped around sheaves and winch drums, inherently limit the weight and length of the perforating gun which can be conveyed therewith. The use of a wireline also restricts the possibility of controlling the movement of the perforating gun other than as dictated by the pull of gravity.
An alternative conveying system known as tubing conveying has therefore been developed. Tubing conveying involves the use of multiple lengths of substantially rigid hollow steel tubing which are interconnected to convey the perforating gun or other similar borehole tools into and out of the borehole. The added mass and mechanical parameters of the hollow tubing affords the possibility of conveying heavier loads into the borehole as well as the possibility of conveying these loads in directions other than the direction of the pull of gravity. Communication with the downhole tool has however been limited to mechanical or hydraulic means such as the controlled rotation of the tubing or the controlled modulation of the fluid pressure within the tubing, etc. Guns which are tubing conveyed typically utilize mechanical means such as manually dropping an object, e.g., a metal bar, into the tubing and causing that object to strike a triggering mechanism on the gun. There are obvious disadvantages to such mechanical or hydraulic means, e.g., relative slippage between the individual joints of the tubing string, leaking joints, etc. In the case of perforating guns and bar drops, falling debris can cause an unplanned detonation, or the debris can block the triggering mechanism and prevent charge actuation. It is therefore extremely difficult with tubing conveyed equipment to reliably afford the controlled actuation of multiple events, e.g., the firing of charges, over a predetermined time period, and in a predetermined order. In addition, it has not been possible to monitor and verify the ocurrence of these events, e.g., whether the charges have been fired as planned. Without such knowledge it is possible in the example of perforating guns that unfired but yet armed charges be returned to the surface in an unexploded condition, and that zones which are thought perforated, are in fact not. The use of tubing conveying has therefore been restricted because of its inherent inability to provide an adequate communication path from the surface of the borehole to the tool contained with the borehole.